Chemical Engineering April 2012 - 17

Process Insight: Comparing Physical Solvents
for Acid Gas Removal
Physical solvents such as DEPG, NMP, Methanol, and Propylene Carbonate are
often used to treat sour gas. These physical solvents differ from chemical solvents
such as ethanolamines and hot potassium carbonate in a number of ways. The
regeneration of chemical solvents is achieved by the application of heat whereas
physical solvents can often be stripped of impurities by simply reducing the pressure.
Physical solvents tend to be favored over chemical solvents when the concentration
of acid gases or other impurities is very high and the operating pressure is high.
Unlike chemical solvents, physical solvents are non-corrosive, requiring only carbon
steel construction. A physical solvent's capacity for absorbing acid gases increases
signifi cantly as the temperature decreases, resulting in reduced circulation rate and
associated operating costs.
Typical Physical Solvent Process
PC (Propylene Carbonate)
The Fluor Solvent process uses JEFFSOL®
PC and is by Fluor Daniel,
S treating
Inc. The light hydrocarbons in natural gas and hydrogen in synthesis gas are less
soluble in PC than in the other solvents. PC cannot be used for selective H2
because it is unstable at the high temperature required to completely strip H2S from
the rich solvent. The FLUOR Solvent process is generally limited to treating feed
gases containing less than 20 ppmv; however, improved stripping with medium
pressure fl ash gas in a vacuum stripper allows treatment to 4 ppmv for gases
containing up to 200 ppmv H2
minimum of 0°F (-18°C) and a maximum of 149°F (65°C).
Gas Solubilities in Physical Solvents
All of these physical solvents are more selective for acid gas than for the
main constituent of the gas. Relative solubilities of some selected gases in solvents
relative to carbon dioxide are presented in the following table.
The solubility of hydrocarbons in physical solvents increases with the
molecular weight of the hydrocarbon. Since heavy hydrocarbons tend to accumulate
in the solvent, physical solvent processes are generally not economical for the
treatment of hydrocarbon streams that contain a substantial amount of pentane-plus
unless a stripping column with a reboiler is used.
Gas Component
H2
DEPG (Dimethyl Ether of Polyethylene Glycol)
DEPG is a mixture of dimethyl ethers of polyethylene glycol.
Solvents containing DEPG are marketed by several companies including
Coastal Chemical Company (as Coastal AGR®
(Selexol). DEPG can be used for selective H2
to yield both a rich H2S feed to the Claus unit as well as bulk CO2
), Dow (Selexol™), and UOP
S removal and can be confi gured
removal.
DEPG is suitable for operation at temperatures up to 347°F (175°C). The
minimum operating temperature is usually 0°F (-18°C).
MeOH (Methanol)
The most common Methanol processes for acid gas removal are
process (by Prosernat). The
the Rectisol process (by Lurgi AG) and Ifpexol®
main application for the Rectisol process is purifi cation of synthesis gases
derived from the gasifi cation of heavy oil and coal rather than natural gas
treating applications. The two-stage Ifpexol process can be used for natural gas
applications. Methanol has a relatively high vapor pressure at normal process
conditions, so deep refrigeration or special recovery methods are required to
prevent high solvent losses. The process usually operates between -40°F and
-80°F (-40°C and -62°C).
NMP (N-Methyl-2-Pyrrolidone)
The Purisol Process uses NMP®
and is marketed by Lurgi AG.
The fl ow schemes used for this solvent are similar to those for DEPG. The
process can be operated either at ambient temperature or with refrigeration
down to about 5°F (-15°C). The Purisol process is particularly well suited to the
purifi cation of high-pressure, high CO2
synthesis gas for gas turbine integrated
gasifi cation combined cycle (IGCC) systems because of the high selectivity for
H2
S.
PM       B R  E I
E S    O G R   C I
    US  
Circle 7 on p. 82 or go to adlinks.che.com/40266-07
Methane
Ethane
CO2
Propane
n-Butane
COS
S
H2
n-Hexane
Methyl Mercaptan
DEPG
at 25°C
0.013
0.066
0.42
1.0
1.01
2.37
2.30
8.82
11.0
22.4
PC
at 25°C
0.0078
0.038
0.17
1.0
0.51
1.75
1.88
3.29
13.5
27.2
NMP
at 25°C
0.0064
0.072
0.38
1.0
1.07
3.48
2.72
10.2
42.7
34.0
MeOH
at -25°C
0.0054
0.051
0.42
1.0
2.35
-
3.92
7.06
-
-
Choosing the Best Alternative
A detailed analysis must be performed to determine the most economical
choice of solvent based on the product requirements. Feed gas composition, minor
components present, and limitations of the individual physical solvent processes are all
important factors in the selection process. Engineers can easily investigate the available
alternatives using a verifi ed process simulator such as ProMax®
which has been verifi ed
with plant operating data.
For additional information about this topic, view the technical
article " A Comparison of Physical Solvents for Acid Gas Removal " at
http://www.bre.com/tabid/147/Default.aspx. For more information about ProMax, contact
Bryan Research & Engineering or visit www.bre.com.
S. The operating temperature for PC is limited to a

http://www.bre.com/tabid/147/Default.aspx http://www.bre.com http://adlinks.che.com/40266-07

Chemical Engineering April 2012

Table of Contents for the Digital Edition of Chemical Engineering April 2012